This invention relates to well drilling operations. More particularly, this invention relates to a system and method for accurately determining the depth of a drilling tool in a borehole.
The use of rotary drilling rigs in drilling oil field boreholes is presently the standard in the art. In rotary drilling, a power rotating means delivers torque to a drill pipe (a plurality of which form a "drill string") via a kelly and a rotary table. The drill pipe or string in turn rotates a bit which drills the borehole through the subsurface formations. Drill strings are supported for up and down movement by a drilling mast located at the earth's surface. A drill line (or "cable") supported to the drilling mast and coupled to the drill string is used on conjunction with a rotating drum to facilitate the up and down movement. The drill line is anchored at one end, called the dead line anchor, which is typically located adjacent a leg of the drilling mast. The drill line extends from the anchor upwardly to a crown block formed of a plurality of rotatable sheaves supported on top of the upper end of the drilling mast. The drill line is reeved about the sheaves in the crown block and extends back and forth between the sheaves of the crown block and rotating sheaves in the travelling block until the desired number of sheaves have the drill line cable received thereon. The drill line then extends from the crown block downward to the rotating drum (i.e., drawworks). The travelling block is provided with suitable means for removably connecting with the drill string such that it may suspend the drill string in the borehole, or be disconnected from the drill string as desired.
As will be appreciated by those skilled in the art, it is of great importance in the drilling of a well to know the drill bit depth, from which is usually derived the hole depth and the tool depth of measurement while drilling (MWD) tools located along the drill string (the term "MWD tools" being used in the broad sense to include logging while drilling and other measurement tools). The drill bit depth is typically determined by a combination of keeping a tally book indicating the lengths of each piece of pipe inserted onto the drill string, and by monitoring the length of drill line being let out during the drilling operation over the length of the new pipe portion. Inaccuracies often arise however. The most simple mistake is an inaccurate measurement or notation of the length of a particular pipe. Another mistake occurs during replacement of the drill bit when the drill string must be disassembled and reassembled. In reassembly, different pipes of different lengths then originally utilized might be used, or the drill stirng might be reassembled in a different order. Also, over the length of a single pipe, inaccuracies arise because the monitoring of the drill line is actually accomplished by monitoring the rotation of the drawworks. However, because the drill line cable stretches over time, and because the drill line is wound around the rotating drum in layers, the rotation of the drum is not easily correlated to the length of drill line being expended.
Further inaccuracies occur during the procedure used for adding additional pipe to the drill string. After the travelling block has moved as far as it can downward, and additional pipe must be added, the drill string is raised by reeling in the drill line cable. When the string reaches the desired height, slips are placed in the rotary table to support the drill string while the kelly is unscrewed. On a basis of a second or so, when the slips are inserted, the travelling block continues to move downward and cable is reeled out although the bit is not moving at all. The disparity in movement is due to the release of tension on the cable as the cable is no longer supporting the weight of the drill string. On the other end of the procedure, after the kelly has been unscrewed, swung over to the new pipe, the new pipe has been screwed onto the kelly, and the kelly and new pipe are swung back and attached to the drill string, the slips are removed. When the slips are removed, again misallocations regarding travelling block movement vis-a-vis drill string movement are made with resulting depth determination inaccuracies.
In order to overcome some of the inaccuracies which have been inherent in the measuring techniques, several procedures have been advocated. For example, in U.S. Pat. No. 4,114,435 to Patton et al., it was proposed to measure different travelling block reference points which related to when the cable on the drum reached different layers of unwinding, and then to determine via an equation, the reference points, the rotation of the drum, etc. the location of the travelling block. The Patton et al. patent, however, still provides inaccuracies in that (among other problems) a change of layers does not occur at an exact point but rather over an entire rotation of the drum. Moreover, as the cable ages, it stretches, an account for such a stretching is not made. Similarly, over time, the drum diameter may change due to wear and replacement of the wrapping guide grooves, and this is not accounted for by Patton et al.
A patent to Mikolajczyk, U.S. Pat. No. 4,787,244 proports to automatically determine the drill bit depth by tracking the movement of the cable. Movements of the cable are only tracked when the weight carried by the travelling block exceeds a certain minimum threshold as determined by a tensiometer on the cable. The Mikolajczyk patent, however, fails to account properly for movements of the cable during the slips in and slips out procedure when the transition is made through the threshold set by Mikolajczyk. As will be set forth below, because of the previously unknown physics of the slips in and slips out procedures, errors on the order of three to twelve inches are typically made using the Mikolajczyk procdure each time a pipe is added to the string. Similar errors are inherent in the proposed system of Chan, U.S. Pat. No. 4,616,321.